Abstract: As shown by numerous presentation at a recent NIH workshop entitled ?Defined Reference Diets for Zebrafish and Other Aquatic Biomedical Research Models: Needs and Challenges Workshop? (https://orip.nih.gov/about-orip/workshop-reports), differences in diet are known variables in animal studies. Use of different diets among laboratories using the same model may lead to lack of reproducibility. Similar to zebrafish, researchers utilizing medaka (i.e., Japanese rice fish), and Xiphophorus species (i.e., platyfish and swordtails) still use a wide variety of commercial and ?home- made? diets. Both animal models are used intensively in genetic studies. Inconsistency in animal diet may confound study outcomes. Therefore, it is important to standardize diet in these aquatic model systems to minimize the environmental (e.g., diet) influence in genetic studies. This proposal is part of a multi-laboratory approach to study the same diet among different aquatic models (i.e., zebrafish, medaka, and Xiphophorus). In concert with our colleagues, we propose to investigate the influence of different diet on medaka and Xiphophorus growth, fecundity and overall animal well-being. Specific Aim 1. We will compare the effects of different diets on performance of medaka and Xiphophorus fishes in growth, fecundity, body fat content, and microbiome composition. Specific Aim 2. We will compare the histology of several organs of medaka and Xiphophorus fed by different diets and determine histological baselines for each diet as a reference for differential animal housing.